1. Field of the Invention
This invention relates to a duplex communication control device used in a communication system comprising a plurality of communication stations and a duplex communication network; and more particularly, to such a device which is capable of being used with a distributed control system, which controls process variables, such as temperature and pressure, in such a communication system wherein the various communication stations communicate with each other by token passing.
2. Description of the Prior Art
A communication system, comprising a plurality of communication stations sharing one communication network, is required to control communications in such a manner that communication between the stations is carried out in an orderly manner.
The token passing type of communication control is well known, and is disclosed, for example in U.S. Pat. Nos. 4,058,681 and 4,491,946, wherein a single token permits each station to serve as a master station and to communicate with other stations, using a communication network. The token is successively passed from station to station in terms of token frames in a predetermined sequence. When one communication station receives this token, it controls the entire communication system, and, when communication is requested, communicates with other stations. When there is no request for communication, the token is sent to the next communication station. The cyclic passing of the token frames is managed according to station addresses intrinsic to individual stations. When token frames are passed to the next station, the token frames are normally sent to the station having a station address which is one larger in number than the station address of the present station.
This token passing type of communication control is advantageous in that it enables efficient use of the communication network. Also, communication control is not centralized. Furthermore, the control is not affected by the length of the communication network. In this way, the token passing type of control has various desired features. Thus, such type of control is widely accepted in process control systems.
In process control systems, which are required by necessity to have high reliability, field control stations assigned to control the process and the communication network to which the field control stations are connected are duplexed. Thus, when one station of the duplex fails, the other station of the duplex operates as a backup.
FIG. 1 shows one example of a conventional communication system comprising a plurality of communication stations ST1, ST2 . . . STn interconnected by a duplex communication network comprising lines BS1 and BS2. In this system, if a trouble or fault occurs at point A in one line BS1, and if the token arrives at station ST1, it is assumed that the station begins to send a token frame toward station ST2, for example, using the first line BS1, at first. Since transmission through line BS1 will be unsuccessful due to the existence of the fault at point A in line BS1, station ST1 then transmits a token frame, using second line BS2. The transmission on line BS2 is successful since there is no fault therein.
When the token is passed to station ST1, station ST1 informs each communication station (ST2 . . . STn) connected by the communication network (comprising lines BS1,BS2) that line BS1 has a fault. On receiving this message, each station (ST2 . . . STn) recognizes the existence of the fault in line BS1 and stops communication through line BS1. Subsequently, communication is continued using line BS2.
The just discussed conventional system has the following problems.
(A) Since station ST1 informs the other stations ST2 . . . STn of the failure in line BS1 when the token is passed again to station ST1 after the failure in line BS1 is detected, the other stations ST2 . . . STn cannot recognize the failure in line BS1 until the token is again passed to station ST1. PA1 (B) If any one station fails to receive the message of the fault, such station will not know of the fault and will continue to act as though such fault did not exist. PA1 (C) If any station does not recognize a line fault, then that station will repeat the operation for detecting line fault whenever the token is passed to that station. The other stations will thus receive repeated failure signals for the same fault. This will cause performance deterioration of the system. PA1 (D) When a line has a fault, if a new station is connected later to the network, then the new station will be unable to recognize the line fault at the time of the later connection.